1. Field of the Invention
The present invention relates to an optical writer and an image forming apparatus including the optical writer.
2. Description of Related Art
Traditional optical writers include gradient-index rod lens arrays (Selfoc (registered trademark) lens arrays; SLAs (registered trademark)) functioning as optical devices. The SLAs each include multiple rod lenses. An error in the fabrication or a warp and/or distortion of the SLA due to the mounting of the rod lens array causes a variation in the distances in the optical axis direction between the individual rod lenses and their corresponding luminous points. The SLA thus has an error in the performance of image formation on the image plane, leading to inferior image quality.
In order to solve the above problem, an example improved SLA includes a convergent rod lens array including rod lenses having end faces aligned in the same plane, a restraining plate disposed on at least one side face of the convergent rod lens array, and an adhesive filled between the convergent rod lens array and the restraining plate (for example, refer to JP S63-91602).
In recent years, optical writers have been developed that include organic light-emitting diodes (OLEDs) in the form of light sources of SLAs. In a general LED print head (LPH) including light emitting diodes (LEDs), multiple light source blocks are connected to define a linear luminous unit. This configuration causes uneven illumination due to an irregular arrangement of the luminous points of the light source blocks and/or a misalignment of the light source blocks. In contrast, the OLEDs are integrated into a single light source. This configuration can avoid the uneven illumination, which is the most serious problem in LPHs.
Unfortunately, a substrate of the OLED light source is subjected to high temperature during the fabrication. The base of the substrate thus should be composed of glass having a particularly low linear expansion coefficient. In other words, the OLED light source has the substrate having a significantly low linear expansion coefficient that differs from that of the SLA. In addition, the substrate of the OLED light source and the SLA each have an elongated shape. Thus, especially problematic is the difference in the thermal expansion between the substrate and the SLA in the longitudinal direction in response to a variation in environmental temperature. FIG. 11 illustrates the difference in the thermal expansion between a light source substrate 1 and an SLA 2. In FIG. 11, the length of arrows D1 indicates the thermal expansion of the light source substrate 1, and the length of arrows D2 indicates the thermal expansion of the SLA 2. A difference in the thermal expansion between the light source substrate 1 and the SLA 2 causes displacement of the rod lenses in the SLA 2 relative to luminous points on the light source substrate 1, leading to inferior image quality.
In addition to the difference in the linear expansion coefficient between the light source substrate 1 and the SLA 2, the difference in the linear expansion coefficient between the SLA 2 and a holder retaining the SLA 2 also leads to displacement of the rod lenses relative to the luminous points.
In general, the light source substrate 1 is bonded to the holder in planes substantially perpendicular to the optical axis of the rod lenses. This configuration can achieve high accuracy of the bearing surface of the holder. The light source substrate 1 thus can be bonded onto the holder via thin layers. In contrast, with reference to FIG. 12, the SLA 2 is bonded to opposite inner faces of a square-tube holder 3, which are substantially parallel to the optical axis. Unfortunately, the holder 3 is fabricated through molding and thus has draft angles on the inner faces. The SLA 2 bonded onto such a holder 3 via thin layers causes inclination and/or distortion of the rod lenses, leading to inferior optical performance. The SLA 2 is thus bonded to the holder 3 with an adhesive E1 having a sufficient thickness.
Since the light source substrate 1 adheres to the holder 3 via thin low-stiffness adhesive layers, the longitudinal centers of the light source substrate 1 and the holder 3 are not readily displaced relative to each other in response to a variation in environmental temperature. In contrast, the SLA 2 adhering to the holder 3 via thick low-stiffness adhesive layers can be anchored to the holder 3 at irregular positions because of the uneven application of the adhesive E1 or the build-up of stress by the curing of the adhesive E1. The longitudinal centers of the SLA 2 and the holder 3 thus can be displaced relative to each other because of the difference in the linear expansion coefficient (refer to an arrow D3 in FIG. 12).
As disclosed in JP S63-91602, the restraining plate is bonded to the SLA to prevent a warp and distortion of the SLA in the optical axis direction. JP S63-91602, however, does not mention the linear expansion coefficient of the restraining plate. In other words, the restraining plate cannot compensate for the difference in the linear expansion coefficient between the SLA and the holder, and thus cannot prevent the displacement of the SLA relative to the holder due to the difference in the linear expansion coefficient. Furthermore, JP S63-91602 discloses no technique to dispose the restraining plate uniformly onto the SLA. In other words, an adhesive E2 between the SLA 2 and a restraining plate 4 inevitably has an uneven thickness, as illustrated in FIG. 13. The restraining plate 4 is thus partially peeled from the SLA 2 in response to local shearing stress caused by a variation in environmental temperature, in particular, at a portion including the adhesive E2 thinner than those in the other portions. This phenomenon leads to a warp and/or distortion of the SLA 2.